1. Field of the Invention
The present invention relates to anechoic coatings which enable the absorption of sound waves in a wide frequency band and, if necessary, under high hydrostatic pressures in order to evade sonar tracking for example. When a sound wave, more generally an acoustic wave, reaches a wall, a portion of its energy is reflected by another transmitted portion and a third portion is absorbed in the wall. For a wall of this type to be anechoic, i.e. for it to reflect no portion of the incident acoustic wave, this acoustic wave must be entirely transmitted or entirely absorbed, or it must be divided entirely between transmission and absorption.
2. Description of the Prior Art
It is known that, at the interface of two acoustic propagation media, with an impedance Z.sub.O for the medium in which the incident wave is propagated and Z for the medium receiving this wave, the reflection coefficient on this interface is: ##EQU1## For the energy to be entirely transmitted, Z should be equal to Z.sub.O. This is generally impossible because of the materials in question. These materials cannot be acted upon because one of them is in a natural medium, most usually in water, while the other material is a structural material of a structure such as, for example, the steel of a submarine hull.
In these cases, there is a known method for cutting the wall with an intermediate layer tending to make this wall anechoic which partly satisfies the equation Z=Z.sub.0 and is, furthermore, absorbent.
If the material is homogenous, these two conditions cannot be met in practice. For, if the the material is to be absorbent, it should show losses. In other words, its dissipation factor should be high. Under these conditions, the impedance Z is complex (i.e. there is a phase shift between the pressure and the speed) while the impedance Z.sub.0 is real, at least in the common example of water.
Of course, a complex impedance cannot be equal to a real impedance and the condition of equality of impedances cannot therefore be met.
Furthermore, the absorption of the acoustic waves is defined by an absorption coefficient .alpha. which is related to the dissipation factor by the relationship: ##EQU2## Consequently, between R and o, there is the relationship: ##EQU3##
There is a known method of manufacturing a partially anechoic material by embedding solid particles in a matrix formed of an elastomer material. These heterogeneities thus cause diffusion and the appearance in this material of shear waves, thus increasing the absorption coefficient. However, the anechoic power of a material of this type remains limited because of the relationship between the absorption and reflection coefficients, especially at low frequencies.
There is also a known method of manufacturing a partially anechoic coating in which the energy is dissipated by viscous friction. For this, the wall is provided with conduits perpendicular to it. The most widely known structure of this type is the so-called alveolate structure. The back of these conduits is given compressible volumes which include, for example, a foam material comprising gas-filled cells. Depending on the dimensions chosen, especially the length and diameter of the conduits, a matching frequency is obtained for which total anechoic quality is achieved.
A coating of this type is decribed, for example, in French patent No. 84.05558 filed on behalf of the firm ALSTHOM ATLANTIQUE.
Apart from the fact that the anechoic quality is not sufficient in a pass-band centered on the matching frequency, an anechoic coating of this type is difficult to manufacture and is therefore costly.